1. Field of the Invention
The present invention relates, in general, to phase-locked-loops and, more particularly, to a phase-locked-loop having automatic sweep acquisition circuitry.
2. Statement of the problem
Phase-locked-loops are widely used in electronic circuitry in applications such as satellite communications, navigation systems, and FM communications. Phase-locked-loops provide functions such as phase detection, frequency modulation and demodulation, frequency division and multiplication, filtering, and oscillator stabilization.
In operation, phase-locked-loops (PLLs) compare the output frequency of a voltage control oscillator (VCO) with a reference signal. If the two signals differ in frequency and/or phase, an error voltage is generated and applied to the VCO, causing it to correct in the direction required for decreasing the difference. The correction procedure continues until "phase-lock" is achieved, after which the VCO will continue to track the incoming signal as long as it remains within the "hold-in" range of the PLL.
The lock-in or pull-in range of range of a phase-locked-loop is that range of frequencies, above and below the correct phase locked frequency, where self acquisition to phase lock is internal to the loop. When the absolute value of the frequency difference is greater than the PLL's bandwidth the circuit cannot acquire phase lock.
Typically, such a circuit may employ a sweep acquisition aid to extend the pull-in range. When and XOR gate is used, the phase detector generates a phase error signal that is a series of pulses. The duty cycle of the phase error signal indicates the degree of phase or frequency offset between the reference signal and the output of the VCO.
The phase error signal also includes a significant noise component caused by practical limitations in the circuitry used. The phase error signal is passed through a "loop filter" that removes the noise component. In the case of a digital phase detector, the loop filter must also integrate the phase error signal to change it from a series of pulses to a direct current (DC) voltage. The DC voltage then drives the voltage controlled oscillator.
The loop filter is conventionally realized as an operational amplifier configured as a lead-lag integrator or similar integrating circuit. Thus, the loop filter typically uses only negative feedback in conventional PLL's.
In order to increase the pull-in range of the PLL beyond that of the PLL bandwidth, additional circuitry must be provided to cause a "sweeping voltage" to the input of the VCO which passes the VCO through the natural "lock-in" range. Typically this is accomplished by circuitry that 1) detects when a phase lock condition does or does not exist and 2) provides a ramped voltage to the input of the for sweep acquisition. Alternatively, the ramped voltage may be applied to the input of the loop filter.
Although the additional circuitry works well to increase the pull-in range of frequencies within which the PLL can lock, it is expensive and increases the overall size of the PLL. Moreover, the circuitry required to detect phase lock and switch the sweep circuitry on and off often requires control logic such as a microprocessor that greatly increases complexity and cost of the PLL. The additional circuitry also adds noise to the PLL that, when amplified by the loop filter, reduces overall stability of the output from the VCO.
A further disadvantage of known phase-lock-loops is that functions such as sweep acquisition, lock detection, and sweep termination must be provided by auxiliary circuitry such as lock detectors, sweep generators, discriminators, and microprocessors that add complexity and cost to the system. The additional circuitry may be more complex and may consume more space, power, etc., than the basic loop itself.
Accordingly, a need exists for a phase-locked loop with an automatic sweep acquisition capability. A need also exists for sweep acquisition circuitry for phase-locked loops that requires little auxiliary circuitry while at the same time rapidly achieving phase-lock. An automatic sweep acquisition circuit is also needed that is simple, economical and consumes little power.